Criteria for designing heaters for fluids have long been known. In ordinary circumstances usual engineering criteria can produce a heater that will provide enough heat transfer to produce the desired amount of heated or vaporized fluid at the desired pressure and with the desired amount of superheat. However, in some circumstances the use of ordinary heater design criteria is not successful in producing a useful heater. One typical example where ordinary heater designs are not useful is in the design of vapor generators to drive external combustion engines for highway vehicles.
To be useful as a vapor generator for a highway vehicle, the fluid heater must produce large amounts of super-heated vapor at superatmospheric pressure; it must be capable of producing vapor at working pressure within seconds of starting; it must be small enough to be conveniently carried on a highway vehicle, for example, under the hood of an automobile; and it should be able to use fuel that can be safely carried and used without causing intolerable atmospheric pollution. Thus, the vapor generator must employ an easily carried liquid or powdered solid combustible fuel rather than fuels that must be maintained at high pressure which are too dangerous in highway vehicles, or nuclear fuels which are too dangerous, cause intolerable pollution, and are unavailable because of their strategic value.